Hybrid powertrains make use of two (or more) propulsion systems to propel the vehicle. The primary device is typically an internal combustion engine and the secondary device is typically an electric motor and battery system. The secondary system is usually capable of utilizing energy from storage batteries to provide tractive energy to wheels and provide energy to start the primary device. Additionally, the secondary system is usually capable of absorbing tractive energy from the wheels, such as during a braking event, to charge the storage batteries.
As a result of this architecture, the combustion engine is frequently turned on and off many times during normal operation of the vehicle. The frequent start events present a significant challenge in terms of meeting the product requirements for noise, vibration and harshness (NVH). Of particular concern are the forces applied from the powertrain through the chassis during engine compression events that are the natural result from engines operating the Otto (four-stroke) cycle. These forces occur during both firing and non-firing modes of operation.